in the Foetus of Vertebrated Animals. 297 



was by indulging in theory, without attention to facts, that the 

 older authors were led into error ; and he endeavoured to prove, 

 that if we regard only the appearances which present themselves 

 to us, we cannot refuse our assent to the conclusion to which they 

 lead. He was aware that it would be impossible to disprove, by 

 argument, the assertion which was made by the opposite party, 

 that all the organs of the animal really exist, though circumstances 

 render them invisible in the germ of the egg ; but he endea- 

 voured to establish his own opinion, without opposing their 

 hypothesis, by recording those observations which seemed to 

 him indisputably to prove the formation of a new part in the 

 egg. The part which he first described was the vascular net- 

 work which appears on the surface of the yolk about the 40th 

 hour of incubation. He followed, with great minuteness, the 

 changes by which the vessels of this network are formed, and 

 afterwards extended his observations to the formation of the in- 

 testinal canal, amnios, and some other parts of the foetus. By 

 these observations he shewed very clearly, not only that the ac- 

 tual formation of parts may be observed to take place, but that 

 after they are first formed, they undergo many important changes 

 in their structure before arriving at their perfect state. 



The observations of Wolff, and the conclusions to which they 

 inevitably lead, viz. that during the development of the foetus, 

 its organs are actually formed, and not simply evolved, gave a 

 new direction and increased interest to the study of this subject. 

 This author has since been followed in his investigations by 

 many other observers. The work of Pander *, especially, on 

 the incubation of the egg, together with the observations of 

 Meckel, Oken, Blumenbach, Baer, Rathke, Cuvier, Dutrochet, 

 Serres, Rolando, Prevost, Dumas, and many others, have con^- 

 firmed and greatly extended the system of which Wolff laid the 

 foundation, and have, indeed, created a new branch of physio- 

 logical science, from which our knowledge of the structure and 

 functions of animals may be considerably increased. 



The observations of the above mentioned physiologists tend 

 to show that the Germinal Membrane of the ovum, in which the 



• Beitrage zur Entwickelungsgeschichte des HUnchens im Eie, Wurtzburg, 

 1817 ; also, Journal des Progres des Sciences et des Inslit. Medic, torn. v. 

 p. 30, and in the Journal des Physique, torn. 68. 



JULY SEI'TF.MBKK 1830. U 



